The long-term goals of the Program Project include the development of a detailed mechanistic model for actin filament branching mediated by the Arp2/3 complex and to characterize the role of the Arp2/3 complex in directed cell motility. Here, we aim at further characterizing the structures of key intermediates and their interactions along the activation pathway of the Arp2/3 complex (Aim 1). For Aim 2, we will generate high resolution, three-dimensional (3D) structures of actin networks mediated by the complex in its cellular environment. This should allow us to determine how structural details of the Arp2/3-mediated actin networks are being modified by changes in cellular dynamics. We will apply high-resolution electron cryo-microscopy (cryo-EM), electron cryotomography and image analysis to achieve these aims.